1. Field of the Invention
The invention relates generally to cementing of well bores.
2. Description of the Related Art
A casing string is generally a plurality of pipe segments, or joints, attached together which are run into a well bore and cemented in place. Casing strings are generally run into a well bore from a rig located on the surface. The casing string is normally not rotated or reciprocated during the run into the wellbore as this could cause damage to threaded connections between segments of the casing string. However, on some occasions the casing string may be rotated and reciprocated when run into the wellbore, particularly in the case of horizontal wells where the frictional forces between the casing string wall and the wellbore may be very high which may result in sticking of the casing string or the inability to run the casing string down to the designed depth. Casing string drag may be a serious problem, resulting in slow running in speed, premature sticking of the casing string, helical buckling or even casing string collapse.
Threaded collars are used to connect two segments of the casing string. A casing shoe is generally placed on the end of a casing string. A curable fluid cement mixture is pumped through the casing string, collars and casing shoe into an annulus formed between the casing string and the drilled wellbore. The casing shoe contains a check valve to prevent the backflow of the fluid cement mixture into the casing string.
The cementing of the casing can be a challenge, particularly in wellbores containing long horizontal sections. The casing string tends to lie along the bottom of the horizontal well bore, making it difficult to evenly disburse fluid cement mixture around the casing string. Often, centralizers are spaced along the casing string in an attempt to “centralize” the casing string in the well bore and to facilitate the placement of the fluid cement mixture evenly around the casing string. Specially designed centralizers have been designed to try and “swirl” the fluid cement mixture around the casing in a helical pattern to facilitate the placement of the fluid cement mixture.